The magnetoresistive element includes a reference layer having invariable magnetization, a storage layer having variable magnetization, and a nonmagnetic layer (a tunnel barrier layer) provided therebetween as its basic structure. When the reference layer and the storage layer have the same magnetization direction, the magnetoresistive element will be in a low-resistance state (i.e., a parallel state), and this state is referred to as, for example, the “0”-write state. Further, when the reference layer and the storage layer have different magnetization directions, the magnetoresistive element will be in a high-resistance state (i.e., an antiparallel state), and this state is referred to as, for example, the “1”-write state.
A write operation to bring the magnetoresistive element to the parallel state or the antiparallel state is performed by passing a write current (a spin injection current) to the magnetoresistive element, when, for example, spin-transfer torque (STT) writing is adopted. Here, from the standpoint of using less current, a value of the write current necessary for magnetic reversal of the storage layer should preferably be as small as possible. However, this means that the magnetic reversal of the storage layer becomes easy. In this case, after writing, magnetic stability (retention) of the storage layer is deteriorated.
As can be seen, reduction of a write current necessary for the magnetic reversal of the storage layer and magnetic stability of the storage layer after writing have a trade-off relationship.